Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred beam direction and is focussed at a selected point along the beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focussed reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer.
This form of ultrasonic imaging is referred to as "phased array sector scanning". Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction (.theta.) during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focussed at a succession of ranges (R) along the scan line as the reflected ultrasonic waves are received.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. Whereas the amplitude of the reflected waves is employed to produce black and white images of the tissues, the frequency shift of the reflected waves may be used to measure the velocity of reflecting scatterers from tissue or blood. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over the black and white anatomical image. The measured velocity of flow at each pixel determines its color.
Referring to FIG. 1, a vibratory energy imaging system includes a transducer array 10 comprised of a plurality of Separately driven transducers 2, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 22. The ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer 2 and applied separately to a receiver 24 through a set of transmit/receive (T/R) switches 26. Transmitter 22, receiver 24 and switches 26 are operated under control of a digital controller 28 responsive to commands by a human operator. A complete scan is performed by acquiring a series of echoes in which switches 26 are set to their transmit position, transmitter 22 is gated ON momentarily to energize each transducer 2, switches 26 are then set to their receive position, and the subsequent echo signals produced by each transducer 2 are applied to receiver 24. The separate echo signals from each transducer 2 are combined in receiver 24 to produce a single echo signal which is used to produce a line in an image on a display system 30.
Transmitter 22 drives transducer array 10 such that the vibrational energy produced, e.g., ultrasonic energy, is directed, or steered, in a beam. A scan can therefore be performed by moving this beam through a set of angles from point-to-point rather than physically moving transducer array 10. To accomplish this, transmitter 22 imparts a time delay (T.sub.i) to the respective pulsed waveforms 34 that are applied to successive transducers 2. If the time delay for all transducers is the same, all the transducers 2 are energized simultaneously and the resulting ultrasonic beam is directed along an axis 36 normal to transducer array 10 focussed to a point at infinity. By adjusting the time delays (T.sub.i) appropriately, the ultrasonic beam can be directed away from axis 36 by an angle (.theta.) and/or focussed at a certain depth. The relationship between the time delay T.sub.i applied to each i-th signal from one end of the transducer array (i=1) to the other end (i=n) is given by the relationship: ##EQU1## where x is the distance of the center of transducer 2 from the center of the transducer array, .theta. is the transmit beam angle, c is the velocity of sound in the object under study, and R.sub.T is the range at which the transmit beam is focussed.
The time delays T.sub.i in Eq. (1) have the effect of steering the beam at the desired angle .theta. and focussing it at a fixed range R.sub.T. A sector scan is performed by progressively changing the time delays T.sub.i in successive excitations. The angle .theta. is thus changed in increments to steer the transmitted beam in a succession of directions. When the direction of the beam is on the other side of axis 36, the timing of pulses 34 is reversed, but Eq. (1) still applies.
The echo signals produced by each burst of ultrasonic energy reflect from objects located at successive positions (R) along the ultrasonic beam. These are sensed separately by each transducer 2 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range (R). Due to the differences in the propagation paths between a reflecting point P and each transducer 2, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. Receiver 24 amplifies the separate echo signals, imparts the proper time delay to each, and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from point P located at range R along the ultrasonic beam oriented at the angle .theta.. Demodulation can occur either before or after the individual received signals are summed together.
To simultaneously sum the electrical signals produced by the echoes impinging on each transducer 2, time delays are introduced into each separate transducer channel 110 of receiver 24 (see FIG. 2). The beam time delays for reception are the same delays (T.sub.i) as the transmission delays described above. However, the time delay of each receiver channel is continuously changing during reception of the echo to provide dynamic focussing of the received beam at the range R from which the echo signal emanates. The exact equation for the receive time delay T.sub.d imposed on the signal received by each transducer element is as follows: ##EQU2## where t is the elapsed time after transmission of ultrasound from the center of the transducer array, c is the velocity of sound in the object under study, .theta. is the beam angle, and x is the distance of the center of the receiving transducer from the center of the transducer array.
Under direction of digital controller 28, receiver 24 provides delays during the scan such that steering of receiver 24 tracks the direction .theta. of the beam steered by transmitter 22 and samples the echo signals at a succession of ranges R and provides the proper delays and phase shifts to dynamically focus at points P along the beam. Thus, each emission of an ultrasonic pulse waveform results in the acquisition of a series of data points which represent the amount of reflected sound from a corresponding series of points P located along the ultrasonic beam.
Display system 30 receives the series of data points produced by receiver 24 and converts the data into the desired image.
Referring to FIG. 2, receiver 24 comprises three sections: a time-gain control section 100, a receive beam forming section 101 and a mid-processor 102. Time-gain control (TGC) section 100 includes a respective amplifier 105 for each of the receiver channels 110 and a time-gain control circuit 106. The input of each amplifier 105 is connected to a respective one of transducers 2 to amplify the echo signal which it receives. The amount of amplification provided by amplifiers 105 is controlled through a control line 107 that is driven by TGC circuit 106, the latter being set by hand operation of potentiometers 108. As the range of the echo signal increases, its amplitude diminishes. The time interval over which the echo signal is acquired determines the range from which it emanates, and this time interval is divided into segments by TGC circuit 106. The settings of the potentiometers for each segment are employed to set the gains of amplifiers 105 during each time interval so that the echo signal is amplified in ever-increasing amounts over the echo signal acquisition time interval.
The receive beam forming section 101 of receiver 24 includes separate receiver channels 110. Each receiver channel 110 receives the analog echo signal from one of amplifiers 105 at an input 111 and produces a stream of digitized output values on an I bus 112 and a Q bus 113. Each of these I and Q values represents a demodulated sample of the echo signal envelope at a specific range (R). These samples have been delayed such that when they are summed at summing points 114 and 115 with the I and Q samples from each of the other receiver channels 110, the summed signals indicate the magnitude and phase of the echo signal reflected from a point P located at range R on the steered beam (.theta.). Alternatively, the demodulation can be performed after the individual received signals are summed.
Mid-processor section 102 receives the beam samples from summing points 114 and 115. The I and Q values of each beam sample are signals representing the in-phase and quadrature components of the magnitude of the reflected sound from a point (R, .theta.). Mid-processor 102 can perform a variety of calculations on these beam samples, depending on the type of image to be reconstructed.
A color flow processor 123 located in mid-processor 102 receives the I and Q values of each demodulated and focussed echo signal sample from summing points 114 and 115, and produces a flow value at the mid-processor output 121. This flow signal controls the red, green and blue display colors, and is applied to display system 30 along with the magnitude M for the same point. The color indicated by this flow value is a function of the velocity and direction of flow, and possibly the flow variance or power, as measured by color flow processor 123.
Thus, conventional ultrasound color flow imaging systems are able to image moving materials, such as blood flowing in the body, by relying on the fact that echoes returning from these moving objects are Doppler shifted. One major limitation of conventional flow imaging techniques, however, is that the measured Doppler shift is proportional to only the velocity component along the direction of the ultrasound beam and not the true velocity. Therefore, the displayed velocity is underestimated by the cosine of the angle between the ultrasonic beam and the fluid flow, and could be missed entirely if the angle is too large.
Several methods have been explored to address this problem. Among these methods are triangulation techniques, wherein the velocity of a region of interest is measured from two different directions, and the two velocity estimates are geometrically combined to yield the total velocity magnitude and direction. This has been accomplished in a number of different ways. A paper by Tamura et al. entitled "Quantitative Study of Steady Flow Using Color Doppler Ultrasound", Ultrasound in Medicine and Biology, Vol. 17, No. 6 (1991), discloses the use color flow frames acquired by steering a linear array of transducers at two different angles. A disadvantage of this system is that two measurements are made at different times, and one must assume that the flow dynamics is unchanged during the interval of time between the measurements. In pulsatile flow studies, the flow stationarity time is generally considered to be on the order of 10 msec, so that separately steered frames acquired in times greater than this cannot be used.
In order to accomplish simultaneous measurement of multiple velocity components, other methods involving special-purpose transducer setups have been proposed. A paper by Overbeck et al. entitled "Vector Doppler: Accurate Measurement of Blood Velocity in Two Dimensions", Ultrasound in Medicine and Biology, Vol 18, No. 1 (1992), reports on the use of a combination of one transmitting transducer flanked by two receiving transducers. The disadvantage of this system is that only the region of interest at the intersection point of the lines of sight for the three transducers is interrogated. Therefore, although this method has promise for a Doppler application, it cannot create a complete image for the color flow imaging application.